We use the murmur server on a linode, so it's totally free (if you have a linux server already). Just "apt-get install mumble-server" on ubuntu server.

Still have to use skype for external calls and stuff, but for internal comms we love it so far.

Isn't the "murmur" server that comes from Mumble free to use?

I know it's not free, but you can easily find fully managed mumble hosting for $4 month (for up to 10 people connected at a time). It may not be completely free, but it's pretty damn close.

I use µMurmur from the OpenWRT project.